Foils for copper-clad laminates suitable for printed circuit board production have heretofore been made, for the most part, by electrodeposition. The many advantages of this method, including speed of production, economy and a very well developed technology, are, however, offset to a substantial extent by certain disadvantages. A very important disadvantage is the difficulty of producing pinhole-free foils of less than 16 microns thickness. Another is the inherent environmental impact of electrodeposition practice. While the pinhole problem may be minimized to some degree by electroplating copper on an aluminum carrier surface specially prepared in accordance with the procedure described in U.S. Pat. No. 3,969,199 to Berdan and Luce, it is at the expense of substantially increased processing complexity and cost. In the Berdan et al. process in order to obtain a peel strength that is not in excess of about 2.0 lb./in. the aluminum carrier surface is first given a zincate coating, the zincate coating is substantially all removed by contact with acid and then overplated with a metal, such as zinc.
These shortcomings of the prior art can be avoided through the use of the inventions disclosed and claimed in the above-referenced co-pending patent applications. They can now also be avoided in still another different way represented by the process of the present invention. Thus, instead of coating the carrier surface with a metal (i.e. zinc or indium as in Berdan et al) or some other release agent prior to laying down the copper film, the copper is deposited directly on the bare carrier surface in such a way as to enable subsequent non-destructive release of the film from the carrier. In other words, this invention enables elimination of the release agent layer while retaining its function. This is accomplished through control of the film bonding action involved in depositing copper on the carrier. This invention, thus, is based upon our new concept of vapor depositing copper on a carrier under conditions of carrier surface temperature, roughness and cleanliness, such as will result in a virtually pinhole-free, high quality, copper film, which is bonded to the carrier but releasable therefrom upon application of peeling or stripping force within an acceptable range. As a general rule, according to this invention, the carrier surface will be clean and free from adhering oil and dirt, which would impair adequate bonding or contaminate the resulting copper film, and will be relatively smooth and free from gross physical irregularities. Further, the temperature of the carrier surface will, in accordance with this invention, be between about 100.degree. and about 250.degree. C. so that bonds which are strong enough but not too strong are formed between the copper film and the carrier surface.
It is a particularly important objective of this invention to be able to utilize commercially available aluminum foil as the carrier sheet in the preparation of the laminate precursor leading to the manufacture of copper-clad assemblies for ultimate usage in the preparation of printed circuit boards.
None of the commercially available foil strong enough to be used for this purpose is completely free of hydrocarbon contaminants. The contamination of aluminum foil begins during the rolling of aluminum billets to produce aluminum foil. The rolling is done in several passes each reducing the aluminum thickness by approximately 50 percent. Each such cold reduction pass is followed by a stress relief anneal to avoid tearing and pitting of the aluminum during the next reduction. Drawing lubricants are used during the cold reductions, the most popular being the combination of kerosene and mineral oil.
Fully soft aluminum foil is desired in many applications and in making foil for such applications the drawing lubricant can be burned off by heating to 300.degree. C. or even higher. However, in the case of aluminum foil for use as a carrier sheet, retention of the mechanical properties is important. Foil thicknesses employed as carrier sheets are of the order of 0.002 inch thick and must be strong enough to be used in roll form in an unrolling and rerolling operation carried on with a fixed tension. In order to avoid tearing, when such carrier sheet material is separated from the completed copper-clad assembly, it is preferable that the yield strength of the aluminum carrier sheet not be less than about 10,000 psi. This latter, therefore, limits the techniques employable for driving off the surface-contaminating hydrocarbons.
Commercially available chemically cleaned aluminum foil is particularly attractive for this commercially available material retains the full hardness of unannealed severely cold worked aluminum foil. The problem remains, however, that aluminum foil so cleaned still retains on its surfaces a minimum of 0.3 .mu.g/cm.sup.2 of hydrocarbon contaminants, which is enough to rule out van der Waals adsorption as a reliable adhesional force between vapor deposited copper and the aluminum carrier sheet on which it is deposited. It is, therefore, necessary to develop other adhesional forces to provide sufficient peel strength in order to have the aluminum foil/copper layer laminate remain intact until it is desired to remove the aluminum by peeling it away. By the practice of this invention it becomes practical to develop such adhesional forces using this commercially available aluminum foil "as received".
We have found in the course of using this invention that electron beam evaporation is an especially satisfactory method of carrying out the copper deposition step, carrier surface temperature being readily controllable in various ways under such condition and a film of requisite thickness being quickly established uniformly over the carrier surface as required. We also contemplate, however, the possibility of ion plating deposition of the copper film, which would involve biasing the carrier and, if required, introducing an inert gas such as argon into copper vapor to establish the necessary ionization effect. Again, in the case of ion plating, carrier surface temperature would be amenable to easy control by a variety of alternatives. Induction (RF) evaporation of the copper instead of electron beam evaporation is also contemplated as a means of producing the vapor phase copper required for physical vapor deposition and again, of course, carrier surface temperature would be readily controllable. If, however, sputtering is the method of deposition to be carried out in the practice of this invention, it will be necessary for special heat removal measures to be taken so as to maintain carrier surface temperature in the range necessary to limit the development of bonding strength to the range we have found to be critical to the new results and advantages of this invention.
It will be understood by those skilled in the art that in whatever manner the invention is carried out to provide the copper film on the carrier surface, thereafter one has the choice of proceeding to produce the laminated body.
In brief summary, then, the method of this invention comprises the steps of forming a copper film on a carrier by vapor deposition of copper directly on the carrier surface at a temperature between about 100.degree. C. and about 250.degree. C., depositing a substrate bonding layer over the copper film, joining the resulting laminate to a substrate by means of strong adhesion between the said laminate and the substrate and, finally, removing the carrier leaving the exposed surface of the unified copper film/substrate. In accordance with the invention of patent application Ser. No. 180,341, the substrate bonding layer may be an electrolytically deposited film, which is preferably of copper and, in any event, is so produced as to have or provide a nodular or dendritic surface formed to promote substrate bonding. Alternatively, the substrate bonding layer may be an electrolytically deposited ultra-thin film of zinc, aluminum, tin or chromium overlaid with an ultra-thin film or silicon dioxide or aluminum oxide, as set forth in patent application Ser. No. 189,003 now U.S. Pat. No. 4,383,003. However, the preferred substrate bonding layer comprises a metal oxide layer and a coupling agent layer as is disclosed and claimed in Ser. No. 406,588.